Periodicals are also called journals or magazines. You will be using several types of online periodicals:
Author (if any). Title of Site. URL (accessed date), other identifying information.
Weisstein, E. W. Molecular Orbital Theory. http://scienceworld.wolfram.com/chemistry/MolecularOrbitalTheory.html (accessed 12/15/03), part of Eric Weisstein's World of Science. http://scienceworld.wolfram.com/ (accessed 12/15/03).